1. Technical Field
The present invention relates to a liquid crystal display (LCD) including a backlight, frontlight, or any other illumination units, and a method for controlling the brightness of the illumination unit included in such an LCD. More particularly, the present invention relates to an LCD and an electronic device that do not use a plurality of interface circuits nor A/D converters, nor experience frequent malfunctions to employ a plurality of photodetectors which produce outputs that require time to reach a predetermined value, which the time is correlated with the intensity of ambient light for automatic control of the brightness of the illumination unit according to the intensity of the ambient light, and to a method for controlling the brightness of the illumination unit included in such an LCD.
2. Related Art
Over recent years the application of LCDs has spread rapidly, not only in information and telecommunications equipment but in electrical equipment in general. Since LCD panels do not themselves emit light, transmissive LCDs equipped with a backlight as illumination unit are much employed. Especially for portable equipment, reflective LCDs which need no backlight have been used, in order to reduce power consumption. But reflective LCDs use ambient light for an illuminating unit and therefore are difficult to view in dark interiors of rooms, etc. Accordingly, in recent times particular progress has been made with the development of reflective LCDs equipped with a frontlight for an illumination unit and semi-transmissive LCDs which possess the capabilities of both the transmissive and reflective LCDs.
Reflective LCDs having a frontlight as an illumination unit display images by lighting the frontlight in dark places, and by utilizing ambient light with the frontlight turned off in bright places. Semi-transmissive LCDs display images by lighting a backlight for an illumination unit and utilizing the transmissive part of the pixel region in dark places, and by utilizing ambient light via the reflective part, without lighting the backlight, etc., in bright places. Thus, such reflective or semi-transmissive LCDs have the advantages of not needing to light the frontlight, backlight, or other illumination units constantly, and of being able to drastically reduce power consumption.
Transmissive LCDs require lower brightness levels of the backlight in dark places, but higher brightness levels thereof in bright places to provide clear visibility.
As mentioned above, the intensity of ambient light affects visibility on the screen of all types of LCDs, and therefore, generally known are such inventions as disclosed in JP-A-2002-131719, JP-A-2003-215534, and JP-A-2004-007237 of LCDs equipped with a photodetector whose output indicates the brightness of ambient light. In such an LCD, the brightness of the illumination unit is controlled according to a detection result given by the photodetector.
For example, JP-A-2002-131719 discloses an LCD that has thin-film transistors (TFTs) which serve as a photodetector on a substrate of the LCD panel. By detecting photo-leakage currents generated in the TFTs, the LCD calculates ambient brightness and automatically controls switching of the backlight according to the brightness. JP-A-2003-215534 discloses an LCD that uses a photodiode for a photodetector and provides temperate-compensated currents to light-emitting diodes (LEDs) which serve as a backlight according to ambient brightness. JP-A-2004-007237 discloses the invention of a mobile terminal that uses LEDs for both a backlight or an operation-state display and a photodetector. In the mobile terminal, switching of the backlight is controlled based on the electromotive force of the LEDs which are generated according to ambient brightness.
When the brightness of an illumination unit is controlled according to the intensity of ambient light as mentioned above, however, ambient light temporarily shaded by a hand or any other object possibly causes false detection of weaker ambient light, resulting in a malfunction. JP-A-2005-121997, therefore, discloses the invention of a method for controlling the backlight of an LCD including a plurality of photodetectors, wherein control of the backlight or any other illumination unit is not enabled until the output from all photodetectors changes to the same extent. JP-A-2007-094097 also discloses the invention of an LCD including a plurality of photodetectors, wherein control of the backlight or any other illumination unit is not enabled until the majority of photodetector outputs change.
In the inventions disclosed by JP-A-2005-121997 and JP-A-2007-094097, analog output devices such as photodiodes and phototransistors are used for a plurality of photodetectors. Outputs therefrom are arithmetically processed, and then control of the backlight or any other illumination unit is enabled based on the arithmetic results. A direct correlation between a current or voltage value output from such an analog output photodetector and the intensity of ambient light makes it easy to determine whether the intensity of ambient light exceeds a predetermined value.
Referring again to JP-A-2002-131719, TFTs used for an ambient light photosensor to detect photo-leakage currents therein (hereinafter called “TFT ambient light photosensor”) show a correlation between the time period taken for a voltage output from the TFT ambient light photosensor to reach a predetermined voltage level and the intensity of ambient light. Use of such a TFT ambient light photosensor essentially requires digital arithmetic processes to determine the intensity of ambient light. The operation principle of such a TFT ambient light photosensor and common detection circuit will be described with reference to the accompanying drawings.
FIG. 10 shows an example of voltage current curves of a TFT ambient light photosensor. FIG. 11 shows an operating circuit diagram of a TFT ambient light photosensor. FIG. 12 shows curves for a voltage at both ends of the capacitor versus time in the circuit diagram shown in FIG. 11 at different brightness levels.
TFT ambient light photosensors and TFTs used for switching elements in an active-matrix liquid crystal display panel essentially have the same structure, which brings the advantage of forming a TFT ambient light photosensor and TFTs used in the active-matrix liquid crystal display panel at the same time. Referring to FIG. 10, such a TFT ambient light photosensor, shaded from light, carries very low dark currents in a gate-off state, and the brighter light shines on the channel, the higher leakage currents it carries.
Referring to FIG. 11, to take advantage of the characteristic of a TFT ambient light photosensor, a constant reverse-bias voltage (e.g., −10V) is applied to a TFT gate electrode GL in the light receptor of the TFT ambient light photosensor LS to create a gate-off state with a capacitor C connected in parallel between a drain electrode DL and a source electrode SL, and a terminal on both of the drain electrode DL and the capacitor C is connected to ground potential. Under this condition, a constant reference voltage VS (e.g., +2V) is applied to both ends of the capacitor C with a switching element S1 turned on. When the switching element S1 is turned off, the voltage at both ends of the capacitor C decreases according to ambient brightness around the TFT ambient light photosensor with the passage of time, as shown in FIG. 12.
As a result, in the TFT ambient light photosensor, the time period taken to decrease to a predetermined voltage V0 after the switching element S1 has been turned off is inversely proportional to the intensity of ambient light. A voltage at both ends of the capacitor C is also inversely proportional to the intensity of ambient light after a predetermined time point t0. Ambient light brightness, therefore, can be calculated by measuring the time period taken to decrease to a predetermined voltage V0 after the switching element S1 has been turned off, or by measuring a voltage at both ends of the capacitor C after a predetermined time point to.
Generally, in order to determine whether the intensity of ambient light exceeds a predetermined value, a sampling hold circuit synchronizing with the on-off state of the switching element S1 is used for data conversion into an analog output voltage, which then undergoes digital arithmetic processes after digital conversion by an A/D converter.
When a plurality of TFT ambient light photosensors are used to control switching of the backlight or any other illumination unit based on the intensity of ambient light and to prevent malfunctions caused by temporarily-shaded ambient light as mentioned above, however, each TFT ambient light photosensor needs a corresponding interface circuit, A/D converter, or the like.